1. Field of the Invention
The present disclosure relates to subsurface geohydrological sampling, and specifically to a method and apparatus for obtaining vadose zone pore liquid samples from the vadose zone beneath the surface of the ground.
2. Background Information
The subsurface of the earth may become contaminated with natural, or more commonly, made-made pollutants. Such contamination may occur in the vadose zone, which is that portion of the subsurface above the natural water table. In the fields of detecting, monitoring, and remediating sub-surface conditions, including the scope and character of contamination, it is often useful to obtain a pore liquid sample from the unsaturated zone of geologic formations to assess the concentration of contaminants of various kinds in the in situ pore fluids. This is usually done by collection of a core sample of the geologic material, or an extraction of the pore liquids using a variety of techniques such as suction lysimeters. However, when a borehole in the unsaturated medium is unstable and the sediments are filled with cobbles, it is often not possible to obtain a core sample of the in situ pore fluids. In addition, the coring process is very expensive compared to the normal drilling of the borehole. Also, for relatively dry geologic media suction, lysimeters are unable to obtain a liquid sample. An unmet need remains for simple and effective methods and means for sampling the pore liquids within the vadose zone.
My previous U.S. Pat. No. 5,176,207, which is incorporated herein by reference, shows the use of a flexible tubular liner with an absorbent outer covering for the collection of pore liquid samples from subsurface boreholes. The liner is installed by eversion down the borehole. The interior fluid pressure of the liner is increased to dilate the liner, thus urging the outer absorber against the borehole wall to allow the absorber to wick the pore liquids from the borehole wall material (i.e., the geologic formation). The absorber continues to absorb the pore liquid until the capillary tension in the absorber equals the capillary tension in the geologic medium. The amount of pore liquids that can be absorbed in the absorbent covering is limited significantly by the capillary tension of the formation. In relatively dry geologic formations (including many vadose zone formations), the method and apparatus of U.S. Pat. No. 5,176,207 absorbs little pore fluid into the outer absorbent layer.